6 Prior to C# 6, you could catch an exception based only on its type. C# 6 introduces exception filters that enable you to catch an exception based on a catch’s exception type and a condition that’s specified with a when clause, as in

catch(ExceptionType name) when(condition)

You also can specify an exception filter for a general catch clause that does not provide an exception type. This allows you to catch an exception based only on a condition, as in

catch when(condition)

In each case, the exception is caught only if the when clause’s condition is true; otherwise, the exception is not caught and the search for an appropriate catch continues.

A typical use of an exception filter is to determine whether a property of an exception object has a specific value. Consider an app that connects to a web server to download videos. Such an app would call methods that may throw HttpExceptions—for example, the web server might not be found, you might not have permission to access the web server, etc. Class HttpException has an ErrorCode property that contains a numeric code, which apps can use to determine what went wrong and handle the exception accordingly. The following catch handler catches an HttpException only if the exception object’s ErrorCode property contains 401, indicating your app does not have permission to access the web server:

catch (HttpException ex) when (exception.ErrorCode == 401)

You might provide several similar catch handlers with exception filters that test for various other ErrorCodes.